In order to lower the threshold current density of a double heterojunction diode laser below the critical limit for CW operation, it is necessary to have the thickness of the active layer on the order of 0.5 microns or less. The usual cleaved and sawed double heterojunction lasers have cross-sectional areas which are typically on the order of 0.2 microns .times. 20 microns. To make the laser beam produced by heterojunction diode lasers compatible with optical systems utilizing round lenses, or other symmetric optical elements, it is desirable to reduce the large dimensional unbalance between active area width and thickness from the order of several hundred to one to as close to a one to one ratio as possible, with a five to one ratio being satisfactory.
Recently, a double heterojunction diode laser was disclosed in which the width of the filamentary area of the active layer was substantially reduced. The disclosed diode laser is called a buried-heterostructure (BH) injection laser since the filamentary active laser region is completely surrounded by a region of lower index of refraction material, that is, surrounded by GaAlAs when the active region is GaAs. The typical fabrication process for the disclosed BH laser is composed of four main steps: (i) a liquid phase epitaxial growth step to produce on a GaAs substrate a first GaAlAs layer, an active layer of GaAs, and a second GaAlAs layer, (ii) a mesa etching step which removes part of the two GaAlAs layers and part of the GaAs layer to define the active filamentary area, (iii) a second liquid phase epitaxial growth step to provide a GaAlAs layer around the mesa to thereby completely surround the active filamentary area with material having a lower index of refraction than that of the active filamentary area, that is, burying or surrounding the active filamentary area with GaAlAs when the active region material is GaAs, and, finally (iiii) a selective diffusion of a p-type dopent (zinc) to provide a p-type channel from the non-substrate end of the device to the GaAlAs layer adjacent the active filamentary area. The last step requires that an apertured masking layer be formed on the non-substrate end of the device with the aperture in precise alignment with the top of the mesa.
The described process has the readily apparent disadvantage of requiring two separate epitaxial growth steps. Another disadvantage is that several layers of varying composition and thickness must be etched, or otherwise removed, and these variances make the etching or removal difficult to control. A further disadvantage is that subsequent to the etching step and prior to the second epitaxial growth, the exposed surfaces of the mesa can get oxidized because of the problem of aluminum contamination with such contamination creating defects in the active filamentary area. Also, the second epitaxial growth can cause melt back of the regions formed by the first epitaxial growth with the likelihood of further defects in the active region.
As noted, the zinc diffusion of the described process to form a non-rectifying channel to the GaAlAs layer on the non-substrate end of the active filamentary area, must be through a masking aperture which is precisely aligned with the mesa top. Such alignment is difficult to maintain because the top of the mesa is hidden by the second epitaxial growth layers and because tolerances of better than a micron must be maintained. In further reference to the zinc diffusion, the diffused region must extend through a relatively thick (5 micron) GaAlAs layer and terminate in a relatively thin (1 micron) GaAlAs layer. If the diffusion is not deep enough a rectifying barrier will be created that will prevent pumpcurrent flow and if the diffusion is too deep the active region (0.5 microns thick) could be penetrated. The zinc diffusion is hard to control due to the different thicknesses between layers, as discussed, and also due to the varying thickness of each layer. Thus, the zinc diffusion must be controlled extremely accurately. Another difficulty with the mesa producing process is that the mesa is a very long, thin plateau which is easily disturbed, that is, chipped or broken off during the subsequent epitaxial growth and zinc diffusion steps.